Spectrophotometric Determination of Cationic Surfactants with Congo Red

A spectrophotometric method was developed for the determination of cationic surfactant.It is based on the discoloration of Congo red(CR)while forming the ion-association complex between the cationic surfactant(CS) and CR in weak acidic HCl-NaAc buffer medium.Spectrophotometric data showed that the maximum discoloration wavelength for CR-cetyl pyridinium bromide(CPB) system was 510 nm and that for CR-Cetyl trimethyl ammonium bromide(CTAB) system was 514 nm.The linear range of the calibration curve,the apparent molar absorptivity and the detection limit for CR-CPB system at 510 nm were 0-4.16×10-5 mol·L-1,1.62×104 L/(mol·cm),and 9.78×10-7 mol·L-1 respectively;and those for CR-CTAB system at 514 nm were 0-3.18×10-5 mol·L1,1.72×104 L/(mol·cm) and 1.04×10-6 mol·L-1, respectively.The method has been applied to the determination of CS in water samples with satisfactory results.